In plastic fabrication manufacturing operations, it is not uncommon for the resin and similar particulate/powdery materials, consumed in the operation to be shipped to the manufacturing facility in heavy containers. These containers are delivered to a manufacturing facility and are stored until required for use in the manufacturing process. When the resin or other particulate matter is required for manufacture, the container is either emptied all at once, or portions of the particulate matter are removed from time to time on an as-needed basis.  The containers are usually too heavy to be lifted manually. Typically a vacuum loader is used to remove the contents.
These plants typically have a supply of “pilot” air, which is at pressure just above ambient, in conduits running throughout the plants. The pilot air is controlled by solenoid or other types of valves and is used for a variety of purposes in the manufacturing plant.
These plants also typically have vacuum lines running through the plant in which relatively low level, i.e. close to but below ambient pressure, is maintained. This moderate vacuum is used for various functions in the manufacturing process.
These plants also typically high pressure air in containers or tanks located at strategic positions within the plant. This high pressure air is typically used for air blown cleaning and sometimes for blow molding, if manufacturing of finished parts is a part of the plant operation. An air compressor may be present if the plant uses significant amounts of high pressure air.
A vacuum loader includes one or more tubes coupled to a vacuum source. The tube(s) is placed within the material storage container and the loader is activated. The resin or particulate material is drawn and conveyed by suction (resulting from vacuum generated by the vacuum loader) from the container to an intermediate location, such as a dryer, prior to being fed to an injection molding machine or an extruder.
Known vacuum loaders filter the air drawn from the material storage container to reduce the presence of contaminants within the particulate resin. It is common for the vacuum source to pull air from the top of a chamber portion of the vacuum loader, to assist in manufacturing the desired product. 
Vacuum is used to convey resin pellets and resin recycle material into position for processing by molding or extrusion. It is also common to place a filter in the vacuum loader, beneath the top chamber, so that all of the air drawn (typically upwardly) through the vacuum loader must pass through the filter. As a result, the air drawn through the top chamber of the vacuum loader is largely free of dust particles and other contaminants. When the vacuum drawing stops, however, the dust and contaminants remain, clogging the filter. This reduces the quantity of air that may be drawn by vacuum through the filter when the system resumes operation. It also compromises the level of suction furnished by the vacuum source.
To overcome this, it is known to blow compressed air downwardly through the filter, in a direction opposite to the direction in which the vacuum is drawn when the system operates. This “blast” of compressed air is typically provided by an air accumulator in conjunction with a solenoid operated valve. A typical accumulator includes an associated reservoir for accumulating a large volume of compressed air within a reservoir space adjacent to the filter. A solenoid actuated valve is positioned between the reservoir and the filter. When the valve is in a closed position, pressurized air accumulates within the reservoir of the air accumulator. Upon opening the solenoid valve, compressed air within the air accumulator, being exposed to the vacuum environment in the vacuum loader, evacuates the accumulator as an air blast, which is directed downwardly through the filter. The air blast applies more air to the filter at a greater pressure, for a longer time period and in a direction opposite that of the air drawn through the filter during normal operation. This reverse flow of air against the normal direction of flow of air drawn by the vacuum source cleans the filter by blasting the dust and contaminants off the filter. Without such an accumulator, the volume and pressure  of air available to blow dust off the filter in the vacuum loader is limited by the amount of air that can flow through the pilot air supply line.
A solenoid operated valve and an air accumulator provide an improvement over other known vacuum loaders and filters that do not have such components. The resulting improvement however, has several limitations. For example, the resulting air blast from the accumulator acts only on a single area of the filter. This is because flow of air into the vacuum loader chamber, where the filter to be cleaned is located, is limited by the size, namely the cross-sectional area, of the internal orifice of the actuating solenoid, through which the “cleaning blast air” must pass. Even when using an accumulator having an associated reservoir, the resulting air blast is limited by the cross-sectional area of the passageway through the solenoid valve, thereby only clearing a correspondingly sized area on the filter; the remainder of the filter is not cleaned.
While simply providing a larger solenoid valve is a possible solution, there are significant cost increases associated with larger solenoid valves. Costs associated with providing a solenoid valve large enough to cover the entire filter is prohibitive. Even with this approach, there is still reduced effectiveness of the vacuum source due to the remaining clogged portions of the filter, when a solenoid valve having a passageway with a cross-sectional area less than the area of the filter is used.
A second approach to this problem is to provide multiple outlets for the air blast against the filter using multiple reservoir chambers and/or multiple solenoid valves. However, this does little to improve the situation. The available “plant” air flow in modern  plastic resin processing facilities is simply too limited to provide sufficient volume and pressure for a multi-outlet configuration to function effectively.
As is apparent, there is a continued need for a highly efficient device to provide periodic air blasts in sufficient volume, at sufficient force over a sufficient area to effectively clean filter units of vacuum loaders, especially those in plants concerned with conveying granular resin pellets and regrind material to injection molding machines and extruders for fabrication.